Bigamy

I am going through a nasty split with my “ex” and need some advice.
When he married me, he was/ is still married to someone else. I did know about the marriage, they do have a formal seperation papers, but the divorce was never final. He looked up some information on the internet and told me that because it was so long that they had been seperated that the marriage was considered “void” so it was ok for us to get married. I’ve done some research myself, and it seems that he lied.
I want to go after him on the bigamy charge, but since I did know he was “married” do I have a case? He did lead me to believe that everything was ok. I’m just afraid that nothing will be done because I knew of this other wife.
Also, do I need to get an annulment? from what I’ve read (and was told from an attorney I called) I don’t have to do anything because the marriage is “void” since he is still married to someone else.
I do have an attorney since there is a restraining order against him, and we are fighting for custody of our son. She advised me to press charges on the bigmay, but I haven’t had a chance to talk to her at length about this yet, and the curiousity is killing me!
Also we were split up for about a year and he still claimed my daughter from another realtionship on his taxes. Can I get him on tax fraud too?